1. Field of the Invention
The present invention relates to an acquisition method and apparatus for seismic exploration. More specifically, the invention relates to a data acquisition method and apparatus for downhole seismics investigating the strata surrounding a borehole.
2. Description of the Related Art
Seismic exploration methods are well established in the art. They are employed to obtain a broad general information about subsurface strata. The known techniques are generally implemented by utilizing an explosive or vibratory source and seismic sensors which are disposed at multiple locations on the surface of the earth or which are towed by a exploration vessel through the water.
More recently, it has be recognized that a combination of seismic exploration methods with selected borehole, measurements may be utilized to obtain more detailed information about subsurface formations and hydrocarbon reservoirs. These methods are often referred to as borehole or downhole seismics, particularly single well seismics and cross-well seismics. They involve the placement of seismic sensors in the borehole, which record the seismic wave field due to a seismic source at another location in the same borehole or in another borehole.
In most applications of downhole seismics, the seismic wave field of greatest interest is the wave field propagating in the rock, i.e., the wave field which would be present in the rock at the sensor location if the borehole itself were absent. Thus, parts of the seismic wave field which are only due to the presence of the borehole itself are usually regarded as noise. In particular, it is desirable to design downhole receiver tools so as to minimize their sensitivity to energy propagating primarily within the fluid-filled borehole. Examples of such waves are known tube waves or low frequency Stoneley waves.
It had been found that the use of hydrophones mounted in elongated cables, so-called "streamers", placed downhole in a fluid-filled well, are particularly responsive to tube waves. In those cases the detected seismic signal is dominated by the tube wave energy, obscuring arrivals representing seismic waves that have undergone reflection from a reflector in the subterranean formation.
A method for attenuating the tube wave signal in a streamer configuration is known from U.S. Pat. No. 4,789,968. The streamer houses pairs of orthogonally mounted hydrophones, thereby permitting the selective detection of waves incident from any direction of interest.
In other methods the hydrophones have been replaced by downhole wall-locking geophones. These geophones are positioned in direct contact to the borehole wall, typically by elements that may be extended outwardly to the borehole wall from a sonde suspended in the borehole fluid at the end of a wireline. A typical example of this technique is described in the United States patent U.S. Pat. No. 5,212,354.
It has been further proposed in U.S. Pat. No. 4,078,223 to mount geophones in a seismic detector cable and oriented in different directions perpendicular to the cable's longitudinal axis. In operation, the geophones that happen to be vertically oriented at any given moment is sensitive to vertical waves. Those geophones which happen to be horizontally oriented at a given moment are disabled by gravity operated switches or the like.
In view of the known arrangements and methods for detecting seismic signals with a downhole receiver, it is the object of the invention to provide a receiver arrangement that is sensitive to the wave field of interest but reduces the noise caused by symmetric waves guided by the borehole.